1. Field of the Invention
The invention relates to diluting engine exhaust or other gases that contain small particles.
2. Background Art
In order to characterize engine exhaust particulate matter (PM) emissions, it is required that the engine exhaust be diluted. Because the engine emits a high concentration of particles, the engine exhaust must be diluted with a high dilution ratio (typically greater than 100:1) to reach the measurable range of certain particle number instruments. The number concentration of engine exhaust particles may vary in a wide range. This is due to the variety of engine technologies and operating conditions. To satisfy these requirements and obtain accurate results, wide range dilution capability is required.
The typical traditional partial flow diluter controls dilution air flow and total mixture flow with mass flow controllers. The sample flow is calculated by subtracting the dilution air flow from the total mixture flow. The dilution ratio is calculated by dividing the total mixture flow by the calculated sample flow. At low dilution conditions, this approach provides an accurate dilution ratio calculation.
As the dilution ratio increases, the accuracy of the calculated dilution ratio drops due to the uncertainties of the total flow and dilution air flow measurements. This results in inaccurate characterization of the emissions. Therefore, use of dilution systems employing the typical traditional partial flow diluter has been limited to somewhat lower dilution ranges, such as 40:1 or lower.
For the foregoing reasons, there is a need for an improved diluter.